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In the end what changed his mind was the prospect of choosing between doing nothing at all or something far more dangerous. The first was not an option for reasons of international relations. He was at pains to point out Australia would be left to take up the slack if New Zealand pulled out. But the United States was the major factor - it recently called for the 60-odd countries taking part in its fight against Isil to do more. That meant doing less was not an option.

For domestic reasons, doing more was not an option, either. There is strong public backing for the deployment so far but the last thing Mr Key needs with an election looming is fatalities.

What he has settled on is doing the same thing but adding an extra element of training to help ensure the Iraqi forces can hold the areas they have won back from Islamic State.

He denied the recent success of Iraqi forces recapturing significant chunks of Fallujah was a factor in the timing of the announcement, but it does not hurt.

After all, it was the Fallujah battle Labour leader Andrew Little set as his litmus test for the success of the training mission. Defence Minister Gerry Brownlee's attempts to get Mr Little on board by taking him to Taji had limited success. Mr Little remains opposed to the deployment and to extending it.

The reason that matters more now is because extended deployment will last until November 2018 - and there is an election between now and then. That leaves Mr Little facing questions as to whether he would immediately recall the troops if Labour is in Government after 2017. He is in the tricky position of insisting he supports the troops' work while questioning whether it is effective.

Mr Key is betting he will be on the right side of public opinion. He prefaced the announcement with a lengthy attempt to justify it, pointing to terrorism acts in the US and foiled plots in Australia. He is betting many New Zealanders would be horrified at the thought of doing nothing while this was going on.